The present invention relates to signal separation circuits, and more particularly to a circuit for separating the L+R and L-R components of stereo audio in the BTSC system.
The BTSC system defines a main audio L+R channel up to 15 kHz, a pilot signal at the horizontal sweep frequency of 15.734 kHz, and a double side band, suppressed carrier stereo subchannel symmetrically about a frequency which is twice the pilot signal frequency. To design lowpass filters for this system requires filter performance several orders of magnitude beyond performance adequate for an FM stereo system. The guardbands between the main channel, the pilot signal and the subchannel are much narrower, and a dbx noise reduction compressor is placed in the subchannel, increasing the potential for subchannel to main channel crosstalk. The guardbands between the main channel and the pilot signal and between the pilot signal and the subchannel are only 734 Hz each, with the separation between the main channel and the subchannel being 1.468 kHZ. Thus, a significant factor in channel separation is the subchannel to main channel crosstalk.
Subchannel to main channel crosstalk occurs when the lower sideband of the subchannel leaks into the main channel due to inadequate lowpass filtering of the subchannel audio. This crosstalk is nonlinear, i.e., it is highly offensive to the ear because it is not harmonically related to the main channel signals. Additionally when the signal levels are low but the program material contains substantial L-R content, the dbx noise reduction compressor can cause subchannel levels to be 20-30 dB higher than main channel levels. When this occurs the main channel has negligible ability to psychoacoustically mask the crosstalk. Further the greatest gain is likely to be produced at high frequencies--the very frequencies that appear at the edge of the lower sideband and which are most likely to cause audible crosstalk.
High performance filters which are adequate for the BTSC system are quite complex, one such having as many as 29 poles of filtering overall, are expensive and require great stability. If the filter is too abrupt at cutoff, the result is ringing coming out of the filter.
What is desired is a stable, simple circuit for separating the L+R and L-R components of the BTSC signal while still eliminating crosstalk between the main channel and the subchannel.